Magnetic recording media generally include a binder dispersion layer comprising a binder and a pigment overlying a substrate, wherein the pigment is dispersed within the binder. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 85% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable.
There are a number of reasons for using as little dispersant as possible. Dispersants tend to soften binder systems, decrease their toughness, and adversely affect their cure. Binders without dispersants can be more readily and reproducibly prepared. Further, excess dispersant may bloom from a cured binder system over time, leading to contamination of a recording head or the like, causing a change in the physical or chemical characteristics of the media.
To help alleviate these problems with added dispersants, binder compositions having internal (chemically bound) dispersants have been developed. Such compositions comprise polymers with functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant or, in some instances, no dispersant is needed for dispersion of magnetic pigment in the binder. However, in spite of these improvements, a higher degree of pigment-polymer interaction is desired, particularly in the case of pigments exhibiting small particle size, such as barium ferrite.
Recently, sulfonated hydroxy-functional polyurethane-containing binder systems have been described which exhibit excellent pigment loading and magnetic orientation (U.S. Pat. Nos. 5,071,578 and 5,085,941, both assigned to 3M). However, these sulfonated polyurethane binder systems are less effective in dispersing barium ferrite (BaFe) pigments, a class of pigments of increasingly important commercial consideration.
Polyurethane polymers containing pendant carboxylic acid groups are known in the art. However none have been disclosed which combine pKa less than about 1.5 with a level of carboxylic acid functionality of about 0.01 to about 0.3 millequivalents per gram. Polymers formed from anhydrides wherein the acid pKa is greater than 1.5 and/or which have a carboxylic acid functionality of greater than about 0.3 millequivalents per gram would not be useful in magnetic recording media.
U.S. Pat. No. 4,096,127, (to Akzona), discloses preparation of carboxyl-functional polyurethanes by half-esterification of hydroxy groups pendant from the polyurethane backbone with aliphatic or aromatic dicarboxylic acid anhydrides. The resultant ester-acids are converted to salts and used as paper sizing agents. Maleic anhydride (acid pKa=1.8) is mentioned in the text (col.2, line 33) in a list of anhydrides that can be reacted with a hydroxyl functional polyurethane to give an acid functional material. The examples, however, do not involve any acids having pKas less than 1.5. No applications towards magnetic media are taught.
U.S. Pat. No. 4,983,491 (to Fuji) describes polyurethanes which have pendant acid groups used in a photosensitive composition. The acid functional polyurethanes are prepared by the reaction of polyurethanes containing hydroxyl groups with anhydrides. While some of these compositions as described contain strong acid groups such as tetrachlorophthalic acid pendant from the polyurethane backbone, their acid content is specified as 0.9 to 3.07 meq/g. These compositions are said to be useful for purposes of photoimaging but such high acid levels are 3 to 100 times higher than is suitable for magnetic media applications.
U.S. Pat. No. 4,612,244 (to Sony Corp.) discloses the potential use of a metal salt of hydroxyacetic (glycolic) acid as a component of a polyurethane binder for a backside coating of magnetic recording tape. Glycolic acid has a pKa of 3.8. U.S. Pat. No. 4,613,545 (to Sony Corp.) discloses the possible use of chloroacetic acid to produce carboxyl-functional magnetic binders. This would produce a moiety of pKa similar to glycolic acid. U.S. Pat. No. 4,571,364 (to Fuji Photo Film Co.) discloses polyurethane resins for magnetic binders in which lysine or its salts or 2-alkyl-2-carboxy-1,3-propanediols are incorporated into the polymer. Lysine has a pKa greater than 2. The pKa values for 2-carboxy-1,3-propanediols are typically about 4.4. U.S. Pat. No. 4,788,103 (to Fuji Photo Film Co.) describes a magnetic pigment binder comprising a polyurethane containing a polar group such as a carboxylic acid or a carboxylic acid salt of an unspecified type and pKa. A carboxyl-functional polyurethane resin for magnetic media is commercially available from Sanyo Chemical Industries under the trade name T17503. Japanese Patent Applications JP 03 64,310 and JP 03 64,314, both to Sanyo, describe a urethane binder prepared by using, inter alia, dimethylolpropionic acid or its ammonium salt. Dimethylol propionic acid has a pKa of about 4.4.
U.S. Pat. No. 3,460,984 (to Gevaert-Agfa) describes the use of low molecular with carboxyl containing materials as magnetic dispersants. These are prepared in examples by the reaction of phthalic anhydride or trimellitic anhydride with a hydroxyl containing polyester or polyether. Trimellitic anhydride (acid pKa=2.5) and phthalic anhydride (acid pKa=2.8) are given as examples. We have found that these acid moieties, when pendant from a polyurethane backbone, perform far less well in pigment dispersing and wetting functions than acid moieties with pKa less than 2.0.
Copending U.S. patent application Ser. No. 08/054,511 assigned to the assignee of the present invention describes the use of polyurethanes with chelating carboxyl groups pendant from their backbone as magnetic recording media binders. These carboxyl groups are of weaker acid type and function through multiple adsorption onto pigment surfaces. While many pigments interact well with these polymers, we have found that some pigments of weakly basic surface chemistry are not well dispersed by these or most other polymers.
U.S. Pat. No. 5,244,739, assigned to the assignee of the present invention, describes the use of vinyl polymeric macromonomer diols in polyurethane polymers useful in magnetic recording media. The patent contains no disclosure of strong acid wetting groups pendant from the polyurethanes.
Blends of polyurethanes with vinyl chloride resins are known in the magnetic pigment binder art. Vinyl chloride resins containing polar functionality, especially sulfonate or ammonium salts, are capable of producing good dispersions by themselves, but these resins generally lack sufficient toughness and flexibility for most product applications. Blending of these vinyl chloride resins with polyurethane resins improves toughness of the resulting binder but care must be taken to preserve dispersion quality. Commercially available carboxyl-functional polyurethanes are compatible with vinyl chloride resins but the known carboxyl-functional polyurethanes interact weakly with pigments and produce relatively poor dispersions by themselves and, in general, they degrade the dispersing capability of ammonium or sulfonate functional vinyl chloride resins when blended therewith.
Blends of polyurethanes with nonhalogenated vinyl copolymers are described in copending U.S. patent application Ser. No. 08/054,312, assigned to the assignee of the present invention, which describes their use as magnetic binders. Polyurethanes are needed as blend resins which do not degrade the dispersing capability of the non-halogenated binders when difficult to disperse pigments like some barium ferrites are used.
As can be seen from this review of known references, a variety of carboxylated polyurethanes have been used to prepare magnetic dispersions. All of these materials are binders with weakly acidic carboxylic acid groups of pKa greater than 2, with the majority of examples having pKa values greater than 4. For some weakly basic pigments, these weak acidic polymers are inadequate to provide good dispersions.